There is a known force sensor having a function of outputting a force applied in a predetermined axial direction and a torque applied around a predetermined rotational axis as an electric signal (for example, Patent Literature 1). In addition to being widely used for force control of industrial robots, the force sensors are also adopted in life supporting robots in recent years, leading to demands for higher safety. The conventional capacitance type force sensor, however, has a concern that, an electronic circuit including a mechanism portion, a capacitance detection unit (force detection unit), and a microcomputer, might fail by condensation, impact, overload, or mixing of foreign matter between a pair of parallel flat plates constituting the capacitive element. In particular, due to flexibility of the force detection unit of the force sensor, overload or repeated load would produce metal fatigue. Metal fatigue might generate cracks or the like in an elastic body constituting the force detection unit, leading to breakage.
As a simple method of judging whether the force sensor is faulty, for example, there is a method of arranging a plurality of (for example, three) force sensors described in Patent Literature 1 in parallel and evaluating a difference between output signals of individual force sensors. In this method, three output signals are compared two by two, and when the difference between the output signals of the two force sensors is within a predetermined range, it is judged that the force sensor functions normally. When the difference does not exist within the predetermined range, it is judged that the force sensor is not normally functioning (faulty).